Au-free Ohmic contacts for AlGaN/GaN based HFETs have advantages over conventional Ohmic contacts. Conventional Ohmic contacts in AlGaN/GaN based HFETs use titanium (Ti) and aluminum (Al), along with cap layers including gold (Au) on Lop and a barrier layer such as Ni, Ti or Pt under. Cap layers with Au are supposed to reduce contact resistance, but the role of top layers, particularly Au, is not well understood, as reported by A. N. Bright et al “Correlation of contact resistance with microstructure for Au/Ni/Al/Ti/AlGaN/GaN ohmic contacts using transmission electron microscopy” in Journal of Applied Physics, Vol. 89, No. 6, page 3143-3150.
Typical annealing temperatures for conventional Ohmic contacts are quite high, and the surface morphology is not smooth. Even with a barrier layer underneath, Au spikes may still punch through the metal stack underneath and may reach the metal/semiconductor interface, which results in low reliability. Such has been observed, as reported by A. N. Bright et al., “Correlation of contact resistance with microstructure for Au/Ni/Al/Ti/AlGaN/GaN ohmic contacts using transmission electron microscopy” in Journal of Applied Physics, Vol. 89, No. 6, page 3143-3150.
Au-free Ohmic contacts for AlGaN/GaN based HFETs and/or MISHEMTs on Si substrates have been described. For example, Hiroshi Kambayashi et al. describe “Improving the Performance of GaN Power Devices for High Breakdown Voltage and High Temperature Operation” in Furukawa Review No. 29, 2006, page 7-12; and B. De Jaeger et al. describe “Au-free CMOS-compatible AlGaN/GaN HEMT processing on 200 mm Si substrates”, in Proceeding of the 2012 24th ISPSD page 49-52″. These two publications report Ohmic layers that are Ti/AlSi/Mo and Ti/Al/Ti/TiN, respectively.
D. Qiao et al. describe “Ta-based interface Ohmic contacts to AlGaN/GaN heterostructures” in Journal of Applied Physics, Vol. 89, No. 10, page 5543-5546, 2001; however the annealing temperature required is quite high with a rapid thermal annealing (RTA) temperature of 950° C. for 4 minutes. Extreme high annealing temperature may either destroy heterostructures or require a complicated cap layer during annealing.
“Tantalum-based ohmic contacts for nitride semiconductor transistors” in Semiconductor Today Compounds & Advanced Silicon Vol. 6 Issue 3 April/May 2011 describes using a lower temperature than the Al melting temperature of 660° C. for annealing; however, obtaining acceptable results using the described technique may be challenging to repeat and it may be difficult to obtain an Rc lower than 1 Ωmm.
S. H. Lim et al. in “Microstructural evidence on electrical properties of Ta/Ti/Al and Ti/Ta/Al Ohmic contacts to n-AlGaN/GaN” Applied Physics Letters Vol. 78, No. 24, page 3797-3799 show the advantages of having Ta at the bottom as compared with having Ti at the bottom of an Ohmic contact layer. They describe a thicker TaN layer formed at the metal/semiconductor interface with Ta at bottom, as compared to the thickness of a TiN layer formed at the same interface with Ti at bottom of Ohmic layer stack. The specific contact resistivity of Ta/Ti/Al is orders of magnitude lower than that of Ti/Ta/Al.
What is needed is an improved Ohmic contact, especially for power switching applications, which use large devices with high breakdown voltage and long term reliability requirements. Low Ohmic contact resistance is also desired. The embodiments of the present disclosure attempt to answer these and other needs.